THIS INVENTION is concerned with improvements in bucket control systems for dragline excavators.
The invention is particularly, although not exclusively, concerned with bucket dump control systems for dragline excavators.
A typical dragline bucket is controlled by two cables or xe2x80x98ropesxe2x80x99xe2x80x94a hoist rope, and a drag rope.
It is noted that where a singular xe2x80x98ropexe2x80x99 is referred to herein, this may, and often does, refer to two or more equalised ropes travelling uniformly and performing identical functions.
The hoist rope is pivotally connected via a load equalizer and hoist chains to trunnions towards and on opposite sides of the rear of the bucket and extends over a sheave at the tip of the excavator boom to the drum of a winch.
The drag rope is coupled via a drag linkage to draw chains in turn coupled on opposite sides of the open mouth of the bucket. Also coupled to the drag linkage is a dump control cable which extends over a dump sheave attached to the hoist load equalizer and back to a mounting lug on a transverse arch extending over the open mouth of the bucket or to the sides of the bucket front. The drag rope extends unsupported between the drag drum of the winch and the drag linkage coupled by draw chains to the front of the bucket.
It is widely held that dragline buckets possess three degrees of freedomxe2x80x94the x and y axes, and the carry angle of the bucket.
In a conventional two rope dragline, the vertical and horizontal positions of the bucket are controlled by the paid out length of the hoist rope and the drag rope. The bucket carry angle is controlled implicitly by the relative lengths of the draw chains, hoist chains, dump rope and connecting links, and the positional masses of the bucket, rigging and payload.
Due to the geometric balance, the carry angle reduces as the bucket moves from the base of the boom to vertically under the boom point. The maximum payload carried by the bucket occurs for only a narrow band of carry angle, with reduced payloads for carry angles higher and lower than this band. Accordingly, the carry angle is at best a compromise between the bucket geometry rigging design and operational requirements.
The dump zone for the bucket is determined by trigonometric stability of the loaded bucket. Generally speaking, at a predetermined distance along the boom, usually more than two thirds of its length, the tensions in the drag rope, draw chain and dump rope, reduce to the point where the dump rope force is no longer sufficient to support the front of the bucket, which rotates about the hoist trunnions to dump the bucket load.
The compromise in bucket carry angle means that efficiencies in the excavation process are lost by bucket spillages, particularly when the bucket is hoisted either close to the base of the boom or more than halfway along the boom. Another limitation of such a rigging design is that generally it is not possible to dump either inside or outside the implicit dump radius controlled by the geometric balance mentioned after.
A prior art two ropexe2x80x94bucket rigging system is described generally in Australian Patent Application No 28097/99 which relates to an improved bucket rigging for a conventional two rope system.
Australian Patent Application No 34502/89 proposes a three cable bucket control system having two hoist ropes and a drag rope. In this proposal, the effective paid out length of the two hoist ropes are independently controllable. This system suggests three controllable degrees of freedom and avoids the compromises with the bucket carry angle of the two rope systems.
The hoist ropes extend over respective sheaves at the tip of the boom, one such hoist rope being coupled via hoist chains to the hoist trunnions of the bucket. The other hoist rope is coupled to the mounting lug on the transverse arch over the mouth of the bucket.
The bucket is moved from a loaded transport position to a dump position by shortening either of the rear mounted or front mounted hoist ropes relative to the other to achieve load dumping from the open mouth of the bucket or rearwardly through the selectively operable hatch. Independent control of the paired hoist ropes is achieved by a radial arm pivoted on the boom support tower. The radial arm has a sheave mounted on the free end over which one of the hoist ropes passes. A hydraulic cylinder is actuable to move the radial arm and sheave whereby one hoist rope is shortened relative to the other.
When the bucket is in a horizontal attitude, the bucket support is represented by a triangulated support structure having one support point at the tip of the boom, another support point at the hoist trunnions, and the third support point at the mounting lug on the bucket arch.
The three rope system is potentially superior to the two rope system in that its effective excavation radius is greater and it permits a greater degree of selectivity in the dump zone position. Also, the spillage resulting from carry angle variations during carrying can be reduced by reducing the angle variation.
Again, while generally effective for its intended purpose, the abovementioned apparatus nevertheless suffers a number of shortcomings.
In particular, in order to dump a loaded bucket, a substantial amount of energy is required to elevate either the front of the loaded bucket relative to the rear or vice versa.
The main problem, however, in a three rope system is that while theoretically providing a greater degree of control over the bucket carry angle over a greater boom slew radius, implementation of a control system to manage the relative rope tensions is considered to be an extremely difficult task.
Accordingly, it is an aim of the present invention to overcome or ameliorate at least some of the shortcomings or disadvantages of prior art dragline excavator control systems.
According to one aspect of the invention there is provided a dragline excavator bucket control system, said system comprising:
a pair of hoist ropes and a drag rope, said system characterized in that said hoist ropes are supported on said boom adjacent a free end thereof at spaced support positions and said hoist ropes are coupled adjacent opposite ends of a dragline bucket whereby said hoist ropes are substantially parallel and the line connecting said boom support points and the line connecting said bucket attachment points are substantially parallel when said bucket is in an optimal carry attitude for said bucket.
Suitably said control system comprises a support system having four spaced support points in side elevation forming a quadrilateral shape.
Preferably, in use, said four points of said support system define a substantially parallelogram shape.
Preferably said bucket, in use, is urged between a transport position and a dumping position by a dumping means, said dumping means being operable by lengthening one of said hoist ropes relative to the other hoist rope whereby gravitational forces cause movement of said bucket between a transport position and a dumping position.
If required, lengthening of one hoist rope relative to the other hoist rope may be effected by separately controllable hoist rope drums.
The separately controllable hoist rope drums may be operated by a common drive.
If required the separately controllable hoist rope drums may be operated by respective drives.
Suitably the separately controllable hoist rope drums may be coupled by a selective engagement mechanism to permit, in use, a predetermined degree of differential relative rotation between said separately controllable hoist rope drums.
The selective engagement mechanism may comprise a clutch mechanism.
Alternatively the selective engagement mechanism may comprise a differential gear assembly.
Alternatively, the bucket, in use, is urged between a transport position and a dumping position by relative movement between spaced upper support positions for said hoist ropes.
If required, a self compensating hoist rope take up system restores the bucket to a carry position under the influence of potential energy stored in said hoist rope take up system.
The self compensating hoist rope take up system may comprise a suspended mass.
If required, the take up system may comprise a spring biassing means.
Alternatively, the take up system may comprise a hydraulic biassing means.
Alternatively the bucket, in use, may be urged between a transport position and a dumping position by a powered system effective to cause relative shortening of one hoist rope relative to the other.
If required, one of said hoist ropes may be shortened relative to the other by a sheave mechanism contactable with said hoist rope.
Suitably, one of said hoist ropes may be shortened relative to the other by selective rotation of a sheave support arm pivotally mounted adjacent a free end of an excavator boom.